Process for improving the permanent shrinkage properties of regenerated cellulose film

ABSTRACT

A PROCESS FOR IMPROVING THE PERMANENT SHRINKAGE AND DURABILITY PROPERTIES OF COATED REGENERATED CELLULOSE FILM WHICH COMPRISES SIMULTANEOUSLY HUMIDIGYING AND SHRINKING THE COATED REGENERATED CELLULOSE FILM IN AN OPERATION SUBSEQUENT TO THE APPLICATION OF THE COATING ONTO THE WEB.

Feb. 23, 1971 D. E. MCDERMOTT ETAL PROCESS FOR IMROVING THE PERMANENT SHRINKAGE PROPERTIES OF REGENERATED CELLULOSE FILM Filed Feb. l5, 1968 2 Sheets-Sheet 1 Byu Q jef/Vr Feb. 23, 1971 D. E. McDERMO-r-r ET AL 3,565,669

PROCESS FOR IMPROVING THE PERMANENT SHRINKAGEPROPERTIES OF REGENERATED CELLULOSE FILM United States Patent Ofiice 3,565,669 Patented Feb. 23, 1971 3,565,669 PROCESS FOR IMPROVING THE PERMANENT SHRINKAGE PROPERTIES OF REGENERATED CELLULOSE FILM Daniel E. McDermott and .lames P. Janosk, Richmond,

Va., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Feb. 13, 1968, Ser. No. 705,158 Int. Cl. B44d 1/44 U.S. Cl. 117--62 4 Claims ABSTRACT OF THE DISCLOSURE A process for improving the permanent shrinkage and durability properties of coated regenerated cellulose film which comprises simultaneously humidifying and shrinking the coated regenerated cellulose film in an operation subsequent to the application of the coating onto the web.

BACKGROUND OF INVENTION One of the largest end uses for regenerated cellulose film today is for product packaging.

The substrate regenerated cellulose film employed is generally manufactured by the viscose process which is described in United States Pats. 1,548,864 and 1,601,289 to Brandenberger. `In this process an alkaline, aqueous solution of sodium celluolse xanthate is forced through an elongate orifice in the form of a continuous sheet into a coagulating bath where a coherent web is formed and subsequently regenerated, washed, desulfured, bleached, and softened. The web in this softened state is then dried by a drying method such as taught by Alles et al., U.S.P. 2,115,132; Britton et al., U.S.P. 2,746,167; and Kane, U.S.P. 3,068,529. The dried web of regenerated cellulose generally contains from 3.0% to 7.0% moisture based on the dry cellulose content of the web. In the drying operation many tensile forces are exerted on the web in both the transverse and the longitudinal directions to ensure that the web is flat and transparent when wound on the post-drying mill roll. The web of regenerated cellulose in this state is not generally useful as a packaging film because of its hydrophilic nature and absence of heat sealing qualities. In this state it is designated as plain transparent film in order to differentiate it from other regenerated cellulosic film which is subsequently coated with one or more types of moisture-proofing and heat scalable materials.

Regenerated cellulosic sheet material as it leaves the drying operation possesses properties which limit its utility to ornamental and sanitary protective packing applications. One of the main deterrents in using plain regenerated cellulose as an overwrap film for packaging rigid nondeformable items is the tendency of such film to shrink in one or more directions when it is exposed to cyclic humidity conditions. This often results in undesirable shrinkage of the overwrapped article causing poor package appearance and, in extreme cases, causing the film either to split or to crush the package contents.

Shrinkage in cellulosic film consists of both permanent and reversible shrinkage. Permanent shrinkage is an irreversible shrinkage which dried regenerated cellulosic films often manifest due to age and certain ambient air conditions. It should be distinguished from reversible shrinkage which normally results from gains or losses in sheet moisture. Permanent shrinkage is accentuated by ambient moisture increases and decreases and by tensile forces that are applied to the web or developed within the web during moisture removal or addition processes. Permanent film shrinkage or net film shrinkage can cause physical damage to a package. Most commercially available types of cellophane will have 3.5% or more permanent shrinkage capability and will exhibit a net machine-direction shrinkage when brought to a 60% moisture equilibrium condition.

Plain regenerated cellulose film is normally coated with a transparent, organic, polymeric material in order to irnprove such specific performance characteristics of the web as moisture-proofness, heat-scalability, dimensional stability, and gas permeability. Coating materials containing nitrocellulose and vinylidene chloride copolymers are commonly employed to broaden the end use applications of cellophane in the packaging industry. These materials are first put into solution by use of volatile organic solvents such as acetone, methyl ethyl ketone, ethyl acetate, tetrahydrofuran, and toluene.

The solutions of the organic polymeric coatings are applied to the regenerated cellulose web by passing the continuous web through the coating solution, metering a. given amount of the solution onto the webs surface, smoothing the solution uniformly across the web and drying the web to remove the solvent and solidify the coating material while the web is subjected to tensile forces. Moisture is lost from the substrate during this solvent removal operation and such moisture is replaced by conditioning the coated web in a post-coating, humidifying atmosphere as taught by Charch et al., U .S.P. 1,826,- 698. This humidication process improves the durability properties of the regenerated cellulose web but the gained improvement is limited by the softener additives in the web, by the degree of exposure of the web to the humidified atmosphere and by the magnitude of the tensile forces being exerted on the web during the humidification process.

SUMMARY OF THE INVENTION The salient feature of the process of this invention is the controlled shrinking of the coated cellulose film structure achieved by operating the rolls at the exit of the postcoating conditioning section at a lower peripheral speed than the peripheral speed of the roll at the entrance to the post-coating conditioning section. Strains that are built into the substrate regenerated cellulose web during its wet processing and drying operations are relieved during this film conditioning operation in which it is humidied and simultaneously shrunken preferably at least 0.5% in its machine direction. The conditioner relaxation attainable in any specic web substrate is dependent on the shrinkage capabilities of the substrate web entering the coating operation.

The process of this invention greatly improves the permanent shrinkage and durability properties of regenerated cellulose film. Film so produced is advantageously used as the overwrap for packages that are subjected to cyclic humidity conditions during warehousing, shipping, and merchandising.

DESCRIPTION OF INVENTION Reference to the following figures will facilitate the understanding of this invention wherein:

FIG. 1 is a diagrammatic side elevation of a coating and humidifying apparatus that is suitable for carrying out the critical relaxation step of this invention;

FIG. 2 shows typical transverse and machine direction dimensional changes for coated regenerated cellulose webs that have been subjected to cyclic humidification changes after having been subjected to a post-coating drawing operation of 0.4%; and

FIG. 3 shows typical transverse and machine direction dimensional changes for coated regenerated cellulose webs that have been subjected to cyclic humidification changes after having been subjected to a post-coating relaxation operation of 1.5%.

FIG. 1 is an elevation view of one form of apparatus Which is suitable for carrying out the process of this invention. In the embodiment shown in FIG. 1 the uncoated regenerated cellulose film A is unwound from the mill roll 11 and enters the coating tower 12 through opening 13. The film 10 enters the coating compartment 14 and passes through the coating solution bath 15, under the dip tank roll 16, through the doctor rolls 17 and 18 and the coating solution smoothing rolls `19 and 20, and `then into the coating solution drying section 21.

Hot, dry air is circulated through the drying section 21 by air blower 22, air heater 23 and exhaust duct 24. This hot circulating air vaporizes the solvent from the substrate 10 and conveys the solvent vapors through the exhaust duct 24.

The coated film after being dried in the drying section 21 passes into the head roll compartment 25, over the head roll 26 and into the conditioning section 27. Moisture lost in the substrate 10 during the coating and drying operations is replaced in the conditioning section 27 by passing humidified air through the conditioning section as the coated film passes through the same secton. This is accomplished by passing air through the blower 23, humidifying means 29, conditioning section 27 and exhaust duct 30. After being humidified the coated web is passed out of the conditioning section 27, under roll 31, around portions of the chill roll or rolls `32 and onto the mill roll windup 33.

In the preferred process of this invention the peripheral speed of the chill rolls 32 are operated 0.5 to 1.5% less than the peripheral speed of the head roll 26 while the coated `web is passed around the surfaces of the head roll and the chill roll, or chill rolls, while the web span between the head roll and the chill roll is simultaneously humidified to effect a controlled relaxation of the web and a controlled shrinkage thereof. Typical roll speeds that have been used are 1200 f.p.m. for the chill rolls and 1208.4 fpm. for the head roll to achieve a 0.7% relaxation. Higher and lower speeds may of course, be feasible for commercial uses. As speeds vary, residence times must remain constant, however, to the extent that suliicient heat transfer may be effected to relieve internal strains in the web.

The humidification atmosphere for this process is conveniently attained by using saturated steam or slightly superheated steam in the humidifying means 29. Steam having a saturation temperature of about 103 C. or steam at atmospheric pressure having a superheat temperature of about 103 C. to 110 C. can be employed successfully in the process of this invention. Humidifying atmospheres formed by use of hot water sprays and live steam so that the atmosphere has an 80 C. to 90 C. dry-bulb temperature and a 75 C. to 85 C. wet-bulb temperature work equally well.

The relaxation range that provides the improvement in film properties without an adverse effect upon roll formation is from just above 0% to 1.5%. The limits of the range are a function of the characteristics of the base web. Experimental data indicates that the practical limits of the relaxation range are from 0.5% to 1.5% in order to attain the maximum improvement in the web due to the controlled shrinkage without loss of other desirable web characteristics.

The salient feature of the process of the present invention is the controlled shrinking of the cellulose film structure in its longitudinal or length direction preferably between about 0.5% and 1.5% based upon its initial length, by relaxing the film structure and relieving the stresses therein while contained within a humidifying atmosphere. The time of exposure of the film to this atmosphere to effect the 0.5 to 1.5 shrinkage is dependent upon the thickness of' and `the type of coating material on the substrate comprising a cellulose web structure. Experience had indicated that this exposure time varies from approximately 4 seconds to 5 seconds for most commercial film types.

The advantages of the process of this invention are graphically shown by comparing the machine-direction curves of FIGS. 2 and 3. The curves shown in FIG. 2 are typical transverse-direction and machine-direction dimensional changes versus moisture changes for coated film which had been humidified while being subjected to 0.4% draw, or stretching, to insure sheet flatness. It is common practice to employ approximately 0.4% draw in the post coating humidification process in order to assure consistently good subsequent roll formation. The dimensional changes of the film shown in FIG. 2 are determined by recording the dimensional changes which test samples of the film evidence when the samples are slowly humidified in a variable humidity atmosphere up to 60% moisture content state at room temperature and slowly dehumidified back to its original moisture level by reversing the humidity cycle of its atmosphere. It is seen from the TD graph of FIG. 2 that a large transversedirection dimensional change takes place in the web during the humiditication portion of the cycle and that this change is essentially nullified during the dehumidification portion of the cycle.

The main problem is evident in the associated machinedirection (MD) curve. It is seen that the web actually shrinks as its moisture content increases during the humidification cycle and continues to shrink upon being dehumidified. It is this feature of standard coated regenerated cellulose packaging film that is detrimental when used in packaging crushable items such as cigarettes. Packages which are subjected to high humidity atmospheres after being packed in a carton oftentimes are crushed when they reach the point of sale.

The improvement in the dimensional change properties resulting from the use of the process of this invention is shown in FIG. 3. It is seen that there is no significant change in the transverse-direction properties as compared with those shown in FIG. 2 but the original curve for this direction shown in FIG. 2 is such that it would never cause package crushing but merely a loose wrap. There is a significant change to be noted in the dimensional change properties in the machine-direction. It is seen :in FIG. 3 that the dimensional change in the machinedirection upon being humidified is always in the form of an expansion and that the change upon being dehumidified does effect a shrinkage of the web but at a significantly lower lever. The importance of the effect on the machine-direction is that the machinedirection of the film is around the package-the weakest direction-and consequently has the greatest effect upon package crushing.

Most packages overwrapped in cellophane have approximately 1.5% excess film in the overwrap. In addition to this, most package constructions can withstand an additional 0.5 overwrap shrinkage, for a total overwrap shrinkage of approximately 2.0% Without causing visible evidence of package deformation. The coated regenerated cellulose iilm produced by the process of this invention will not deform the package to the degree that conventionally coated and processed cellophane does when subjected to extreme humidity cycles. Additionally, the

lower level of shrinkage of the film produced iby the process of this invention precludes overwrap failures when used to overwrap a crush-resistant package.

The principle and practice of the present invention will now be illustrated by the following examples which are illustrative only and do not in any Way limit this invention.

EXAMPLE I A regenerated cellulose film of 0.0009-inch gage thickness with a 5% moisture content and a 26% softener content was solvent coated with a nitrocellulose coating such as disclosed in Example I of U.S.P. 2,307,057 in a coating apparatus as shown in FIG. 1. The coated web had 3.4gm./m.2 of coating and had a final moisture content of 7.5%.

The coated web was exposed for a period of 4 to 5 seconds in the conditioning section to a conditioning atmosphere of live steam having a superheat temperature of 105 C., then withdrawn from the conditioning atmosphere at a rate of 0.7% less than the feed rate into the section, and wound onto a core.

Film samples measuring 6 x 6 were taken from the wound roll and immersed in water for 24 hours. The samples were then measured and showed a machinedirection `wet shrinkage of 1.04%. The same samples were then dried for 72 hours under ambient room conditions of 75 F. dry bulb temperature at 35% relative humidity and remeasured in the machine-direction. The dry shrinkage amounted to 4.43% in the machine-direction. This exposure of the film to extended immersion in water followed by redrying of the film demonstrated a 30% improvement in the wet-state machine-direction shrinkage properties and an 11% improvement in the redried state machine-direction shrinkage properties when the web was relaxed 0.7% in the post coating operation over such a sample submitted to the same testing but having had a 0.5% draw in the conditioning section.

This test is a severe experiment for coated regenerated cellulose film and is not typical of any end-use applications. It was employed to minimize the effect that the softener content of the web has on the wet and dry shrinkage properties since the residual softener readily goes into solution in the presence of water.

EXAMPLE II The testing procedures and samples from the film of Example I were repeated with the exception that the web relaxation was 1.3%.

Samples were tested as described in Example I and showed a machine-direction wet shrinkage of 0.52% and a dry shrinkage of 4.17%. This exposure of the film to extended immersion in water followed by redrying of the film demonstrated a 65% improvement in the wet-state machine-direction shrinkage properties and a 16% improvement in the redried state machine-direction properties when the web was relaxed 1.3% in the post-coating operation.

EXAMPLE III A mill roll of cellophane was coated in a manner similar to that described in the previous two examples. The first half of this mill roll 'was conditioned as previously described while the web was subjected to a sub-normal 0.2% draw and the second half of the roll was conditioned while the web was relaxed 0.7%.

Film samples measuring 4" x 6 were taken from the first and second halves of the mill roll and were placed in an oven and cycled through the following ambient conditions:

(l) 24 hours at 100 F. and 90% RJH. (Relative Humidity) (2) 24 hours at 100 F. and 20% R.H.

(3) 24- hours at 100 F. and 90% R.H.

(4) 24 hours at 100 F. and 20% R.H.

() 72 hours at 75 F. and 55% R.H.

The samples were then removed from the oven and the machine-direction shrinkage was measured. The shrinkage at `0.2% draw was 3.5% while the shrinkage at 0.7% relaxation was 2.9%. This exposure of the film to cycling, humidified-air conditions demonstrated a 17% improvement in the machine-direction shrinkage properties for film relaxed 0.7% in the post-coating operation as compared -with film slightly tensioned during the postcoating operation.

EXAMPLE IV A third roll was processed and tested as described in Example III with the first portion of the roll subjected to 0.2% draw and the second half of the roll was conditioned while the web was relaxed 1.0%.

The machine-direction shrinkage of the film subjected to 0.2% draw was 3.4% while the machine-direction shrinkage of the film which was relaxed 1.0% during the post-coating operation was 2.6%. This exposure of the film to cycling humidified air conditions demonstrated a 24% improvement in the machine-direction shrinkage properties for film relaxed 1.0% as compared with film processed with a subnormal degree of tension imposed during the post-coating operation.

EXAMPLE V Softened and dried regenerated cellulose film was taken from a cellophane dryer as in Example I and was coated and subjected to normal conditioner drawing of 0.5%. Samples of this film were tested for machine-direction elongation. It was determined that the film produced under these operating conditions had a 15% machinedirection elongation.

Additional film produced under the same operating conditions was coated and subjected to a post-coating treatment of 0.7% relaxation as in Example I. Samples of this film were tested for machine-direction elongation. It was determined that the film produced under these operating conditions had a 15% machine-direction elongation.

Additional film produced under the same operating conditions was coated and subjected to a post-coating treatment of 0.7% relaxation as in Example I. Samples of this film were tested for machine-direction elongation and it was determined that the film produced in this manner had a 20% machine-directional elongation which demonstrated a 33% improvement in this property. Improvement in the machine-direction elongation property of regenerated cellulose film also affects an improvement in the durability properties of the film.

EXAMPLE VI The regenerated cellulose film used in the preparation of Example I was coated to the same coating thickness as the film in Example I using a saran polyester coating such as described in U.S. Pat. 2,570,478 and processed in the post-coating operation under the same conditions as was the lm in Example I.

Permanent shinkage tests were conducted using this lilm. These tests were comparable to the tests performed on the nitrocellulose coated films. These tests revealed that the shinkage properties of post-coating relaxed saran coated film was statistically equivalent to the shinkage properties of post-coating relaxed nitrocellulose coated film.

The test results demonstrate that the web improvements resulting for the use of the process of this invention is independent of the type of coating on the substrate and that the post-coating relaxation process is, therefore, a regenerated cellulose film substrate improvement process. It has been noted, however, that saran polymer coated film produced by the process of this invention yields superior performance characteristics in packaging applications than does similarly produced nitrocellulose coated regenerated cellulose film. This performance superiority is attributable to the greater sealing and scratch-resistant characteristics of Saran polymer coated film.

What is claimed is:

1. In a process for coating a lweb of regenerated cellulose film including applying a coating composition of at least a film forming coating material and a solvent onto the web, heating the coated web while in a circulating air stream to remove the solvent and solidify the coating material, passing the coated web around a driven head roll at a constant rate of speed after the solvent is removed by heat and circulating air and into a humidification chamber, humidifying the coated web, passing the humidied and coated web around a driven chill roll and winding the coated web onto a core, the improvement comprising:

shrinking the coated web in its length direction from 0.5% to 1.5% based on its initial length while it is being humidied by relaxing the coated web andl relieving the stresses therein by maintaining the surface speed of the driven chill roll at a speed from 0.5% to 1.5% less than the surface speed of the driven head roll.

2. The process of claim 1 wherein the web is humidied in an atmosphere of steam at atmospheric pressure having a 3 C. to 10 C. degree of superheat.

3. The process of claim 1 wherein the web is humidied in an atmosphere having a dry-bulb temperature of 80 C. to 90 C. and a lower wet-bulb temperature of 75 C. to 85 C.

4. The process of claim 1 wherein the web is shrunk by from 0.5% to 1.5% while it is being humidied.

References Cited WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner U.S. C1. XR. 

